Use of the global positioning system (GPS) is growing rapidly in both the military and commercial sectors for a variety of applications such as guidance, navigation, search and recovery missions and surveying, just to name a few. It is also known that atmospheric and ionospheric influences of GPS signals (transmitted from the orbiting GPS satellites) cause approximately a 13 meter circular error probability (CEP) in position determining and navigation accuracy. This means that triangulation using GPS signals can generate position coordinates that are off by as much as 13 meters at any given time. However, this error is not acceptable in some applications. One such example is the clearing of littoral obstacles and mines at coastal regions via the use of GPS-guided weapons.
Amphibious assault through coastal regions defended by littoral obstacles and mines is a dangerous mission. The National Security Act of 1947 requires the United States Marine Corps to maintain the capability to effect a forcible entry onto a defended shore by means of amphibious assault. Towards this end, several means are being developed to effect such a forcible entry. One method makes use of precision GPS-guided munitions released from aircraft to strike anti-invasion mines and obstacles placed in shallow water and beach heads by an enemy. These precision, GPS-guided munitions are intended to strike specific lanes of entry and clear them by means of explosive detonation. However, as noted above, current GPS precision in these munitions is limited due to atmospheric and ionospheric influences. Thus, the munitions can only be targeted to an entire area. In general, this means that more munitions will be required than actually necessary thereby requiring more sorties than actually necessary. This increases the risk to military pilots and obviously adds to the cost of the mission.
One approach to dealing with the error introduced by atmospheric and ionospheric influences is disclosed by Schneider in U.S. Pat. No. 5,554,994. Briefly, Schneider discloses a self-surveying relative GPS weapons guidance system in which a ground sensor surveys a plurality of GPS satellites. The ground sensor calculates its location both immediately upon acquiring GPS signals and again after several hours have passed. The difference between the two calculated locations is a GPS error which is then transmitted from the ground sensor. Incoming guided weapons use GPS satellite signals and the ground sensor-transmitted error to navigate more precisely to their aim point.
There are several drawbacks to the system disclosed by Schneider. For example, the navigation processor onboard a guided weapon must first calculate its rough position using the GPS satellite signals and then incorporate the error computation from the ground-based sensor in order to determine its position more accurately. More importantly, the error calculated and transmitted by the ground-based sensor is not the actual error experienced by the guided weapon. This is because the error path from any given satellite to the moving weapon is different from the error experienced and calculated by the ground-based sensor. The most notable cause of this difference is the transmission path length difference. In other words, GPS signal distortion between the guided weapon and a satellite is different than that between the ground-based sensor and the same satellite. Thus, the error transmitted by the ground-based sensor is not the error (correction) needed by the guided weapon to achieve the best navigation accuracy.